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Genetic Heritage

Melissa Holmes

January 1, 2002

Genetic Heritage

Dozens of times you've answered survey questions by identifying yourself as a member of a certain race: Hispanic, Caucasian, African American, American Indian, Asian Indian, Asian, Pacific Islander, Other. Not difficult—you've known since childhood which group you belong to. Surprisingly, however, your genes may disagree.

"People may identify themselves as part of a certain population when they're genetically closer to another," says former Penn State graduate student Carrie Pfaff. As part of her doctoral research under assistant professor of anthropology Mark Shriver, Pfaff developed a method for documenting the gap between ethnic identity and what she calls "biogeographical ancestry." Pinpointing the latter means tracing an individual's genes, through generations of mixing, back to their geographical roots. Whether your ancestors came from the Middle East or Africa, Europe or South America, your genes retain subtle evidence of your origins.

"Overall, there are few major differences between human populations," Pfaff explains, "but there are places in the genome where people differ based on geographical origin." For example, researchers have identified a gene called Duffy (named for the hemophilia patient in which it was first identified) which codes for blood type, similar to A, B, or O. While studying antigens (the body's antibody producers) associated with the Duffy group, geneticists identified a component that provides resistance to malaria. This particular antigen occurs only in people of African descent, an apparent evolutionary response to malaria's historical existence on that continent.

To assemble a group of genes she could use as controls, Pfaff picked out genes like Duffy, which other researchers have proposed as population-specific, then looked for those genes in the DNA of individuals in "parental groups"—groups of people with relatively unmixed ancestry—from Europe and Africa. If the gene was significantly more prevalent in one group than in the others, Pfaff added it to her list of "ancestry-informative markers." Once she had compiled a set of these markers, Pfaff wrote a computer program that would estimate an individual's ancestry by comparing his or her DNA to the list of controls.

For her test group, Pfaff collaborated with researchers at Howard University who are experimenting with the genetics of skin pigmentation in relation to diseases such as hypertension, diabetes, and obesity, which have higher incidences in certain cultures. Her subjects were Howard students, all of whom identified themselves as African American. When Pfaff analyzed these subjects' DNA, however, she found a large amount of variation in the degree of African ancestry, from only 10 percent up to nearly 100 percent.

Biogeographical ancestry is separate from cultural identity, Pfaff stresses. Her concern is that the discrepancy between culture and genetics may distort the results of research, especially research exploring racial predisposition to disease.

Currently, when researchers want to know a test subject's race, she points out, they simply ask. If the study is culturally oriented, that approach is fine, she suggests. However, when the study involves claims related to genetic make-up, genes—not culture—are what is important. "People don't know their molecular ancestry," Pfaff says. "Identifying them as, say, European when they're not could lead to confounding effects."

Precise definition of ancestry is especially important for understanding diseases like hypertension and diabetes, whose causes are difficult to pinpoint, Pfaff says. "These are conditions in which cultural and environmental causes interact with genetics. Without correcting for ancestry, researchers might get a false signal."

She is confident that her new method can help prevent this problem, especially if she can improve its reliability by adding more markers to her control set. Adding markers, however, would require identifying more biogeographical differences, which in turn would require gathering DNA samples from a larger number of people. Because of concerns about potential misuse, she realizes that may be a difficult proposition until better guidelines have been established regarding the use of genetic information.

In the meantime, Pfaff plans to continue her search for ancestry-informative markers with information that is currently available. Her adviser, Mark Shriver, says her method will likely become increasingly important to accurate research. "With multiracial families growing ever more common," he predicts, "we'll be able to tell less and less about a person's genetics by social or physical characteristics."

Carrie Pfaff received her Ph.D. in anthropology in May 2001. Her adviser was Mark Shriver, Ph.D., assistant professor of anthropology in the College of the Liberal Arts, 409 Carpenter Bldg., University Park, PA, 16802; 814-863-1078; mds17@psu.edu.